We aim to measure the average dust and molecular gas content of massive star-forming galaxies ($\rm > 3 \times 10^{10}\,M_⊙$) up to z=4 in the COSMOS field to determine if the intense star formation observed at high redshift is induced by major mergers or caused by large gas reservoirs. Firstly, we measured the evolution of the average spectral energy distributions as a function of redshift using a stacking analysis of Spitzer, Herschel, LABOCA, and AzTEC data for two samples of galaxies: normal star-forming objects and strong starbursts, as defined by their distance to the main sequence. We found that the mean intensity of the radiation field $$ heating the dust (strongly correlated with dust temperature) increases with increasing redshift up to z$∼$4 in main-sequence galaxies. We can reproduce this evolution with simple models that account for the decrease of the gas metallicity with redshift. No evolution of $$ with redshift is found in strong starbursts. We then deduced the evolution of the molecular gas fraction (defined here as $\rm M_{\rm mol}/(M_{\rm mol}+M_⋆)$) with redshift and found a similar, steeply increasing trend for both samples. At z$∼$4, this fraction reaches $∼$60%. The average position of the main-sequence galaxies is on the locus of the local, normal star-forming disks in the integrated Schmidt-Kennicutt diagram (star formation rate versus mass of molecular gas), suggesting that the bulk of the star formation up to z=4 is dominated by secular processes. ; Comment: 17 pages, 14 figures, 2 tables, accepted by A&A